This invention relates to a photoelastic effect measuring device utilized for the stress analysis of a body.
When force is applied to an optically isotropic and transparent body such as glass, plastics, etc., an optical anisotropy is produced, depending on strain or stress, which gives rise to a birefringence. In the case where a stress analysis of a sample is effected by utilizing this photoelastic effect, it requires white light and a monochromatic light having a specified wavelength as light sources and measurement results should be obtained for each of these light sources.
Prior art photoelastic effect measuring devices were provided either with two sorts of light sources, white and monochromatic, or with a white light source and a filter for obtaining a monochromatic light.
An example of the photoelastic effect measuring device is disclosed in U.S. patent application, Ser. No. 711, filed Jan. 6, 1987 by Yoshii et al. In this device a white light source is used and the white light emitted by it is dispersed by means of a grating in order to obtain selectively only a monochromatic light having a specified wavelength.
The grating has a high dispersive power and when white light is reflected by it, a light spectrum dispersed over a wide range of angle is obtained. On the contrary, the dispersive power of a prism is lower than that of a grating and the band width of its light spectrum is considerably narrower than that of the grating. Therefore light energy passing through a predetermined slit, when white light is dispersed by means of a prism, is several tens to several hundreds times as high as when it is dispersed by means of a grating. Further, when a monochromatic light is obtained by a grating and a slit, the light energy is too low and it may be difficult to measure it. In addition, since the width of the light spectrum dispersed by means of a grating is extremely great, it is impossible to fabricate a slit, which can make the light pass through over the whole light spectre.